81 



part of the range from to 4000 fathoms. We have become convinced that many 

 of the plains we find in the Atlantic have slopes, perhaps as small as .05 ft/mile. 

 Fourthly, the reliability and repair procedures must be greatly improved. Many 

 miles of sounding track have been lost while a trained technician was repairing 

 the sounder and many more miles of sounding track have been lost for the lack of 

 a trained technician to make repairs at sea. 



Another geophysical measurement made at sea, of great importance, is 

 the measurement of gravity. Since there is no known variation of gravity with 

 time, measurements at the same location only have to be made as cross checks 

 on the apparatus. At present, the only apparatus capable of measuring gravity 

 at sea is the Vening-Meinesz (1929) pendulum apparatus. The measurements 

 must be made from a submerged submarine to avoid the grosser effects of the 

 acceleration due to surface wave motion. This gear is accurate to +3 milligals 

 (+^003 cm/sec2). Many improvements in this apparatus have been made princi- 

 pally to simplify its operation, adjustment, and data reductions, (Worzel and 

 Ewing, 1947, 1948, 1949) and two major improvements have been made. These 

 major improvements are the use of a crystal chronometer for time measure- 

 ments to replace the springwound chronometers, (Ewing 1937) and the long period 

 pendulum apparatus (Vening-Meinesz, 1941) necessary to measure the second 

 order corrections, whose importance was first pointed out by Browne (1937). 



In the computations it is necessary to make corrections for pendulum 

 amiplitude, air density, temperature, isochronism of ,the pendulums which consti- 

 tue a fictitious pendulunn, chronometer rate, deviation of the pendulum swinging 

 plane from the vertical, second order corrections for horizontal and vertical 

 accelerations, the depth of the submarine, and the Eotvos correction (east-west 

 component of velocity during the observation). For topographic and isostatic 

 reductions the water depth is also required. 



While it is of course desirable to have air-borne gravimeters or surface 

 ship borne or towed gravimeters, it is unlikely that either will be forthcoming 

 in the near future due to the very large acceleration to which surface ships and 

 aircraft are subject. It is possible that a gravimeter for use on submarines will 

 will be forthcoming in the not too distance future. Such an instrument cannot 

 be expected to cut down the observing time much below the present 25 minutes, 

 especially if we strive for 1 milligal accuracy, since 1 milligal vertical accele- 

 ration continued over a period of time of the order of 10 minutes would just pro- 

 duce an observable depth change in the presence of surface waves. Such an in- 

 strument would shorten the time to reduce the gravity value, now 4 hours (1 hour 

 for photographic processing, 3 hours for record reading and computing) to about 

 5 minutes. 



Gravity observations and their associated measurements are of impor- 

 tance to oceanography to determine the density distributions in the crust beneath 

 the ocean, and for the observations of the wave motions in deep water. It would 

 be possible to use the Eotvos effect to determine the East-West current velocity. 



For geophysical purposes existing airborne total field magnetometers are 

 satisfactory (Wyckoff, 1948) (Frowe, 1948). However, since airplanes seldom 

 know where they are at sea to better than 15 or 20 miles, it is also important to 

 have a ship towed magnetometer to determine the relationship of magnetic anom- 

 alies to the bottom topography, as a means of interpreting the airborne data. 

 We have made an adaptation of one of the airborne instruments for this purpose. 

 Since this merely involves replacing the "bird" with a watertight "fish", no in- 

 strumentation details need be given. The biggest problem, now well on the way 

 to its solution, has been that of obtaining a cable able to withstand the strain of 

 towing for thousands of miles in all kinds of weather. 



